Recent developments in screen-printing machinery enable the treatment of web surfaces having widths of five meters or more. As the width of the surface to be coated or impregnated increases, the maintenance of a uniform flow along a discharge slot of an applicator nozzle becomes ever more difficult. The requisite equalization of the supply pressure over the full width of the substrate creates problems especially with fast-running machines designed to apply only a thin layer of the flowable material to the underlying surface, as will generally be the case in the printing of a nonabsorbent substrate. In principle, the term "flowable material" encompasses foaming and nonfoaming liquids of various viscosities, gases and also comminuted matter; for convenience, however, we shall hereinafter refer to that material as working fluid.
The use of positive-displacement pumps for the forced feeding of the working fluid to a discharge slot has already been proposed in conjunction with screen-printing machines in which a wiper in the form of a doctor blade or a roller adjoins that slot to control the layer thickness. Such pumps, however, generally operate with significant pressure variations resulting in objectionable marks on the substrate surface. The utilization of gravity feed from an elevated supply vessel obviates this inconvenience but has the drawback that the channel system of the applicator, conducting the working fluid from its inlet to its outlet, tends to retain substantial portions of that fluid at the end of a printing operation. This requires a thorough cleaning of the channels before the next operation and entails losses of dyestuff which can be relatively significant in the case of short production runs.